Dioxins, the typical example of which is 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), are extremely harmful to human bodies and the discharge thereof to an atmosphere is strongly prohibited. Ministry of Health and Welfare of Japan published "Guideline for Prevention of Generation of Dioxins in Waste Treatment" in January, 1997, in which the concentration of dioxins in flue gases discharged from any newly constructed furnace is instructed to be suppressed to 0.1 ng-TEQ/Nm.sup.3 or less. Environment Agency of Japan designates dioxins as being harmful substances by an amendment of the Atmospheric Pollution Prevention Law of December, 1997 and defines a regulating value for dioxins generated by combustion of industrial wastes as well as municipal solid wastes.
Various methods have been hitherto proposed for decomposing dioxins into harmless substances. Examples of these methods include a combustion method, a melting method, a thermal decomposition method, a photo-decomposition method, an ozone decomposition method, an oxidation decomposition method using hydrogen peroxide, a hydrothermal decomposition method and an alkali decomposition method.
The conventional methods, however, involve problems, because they encounter great difficulties in practicing and because they are not satisfactory from the standpoint of economy.
JP-A-H10-146574 proposes a method of decomposing dioxins, wherein a dioxin-containing fly ash is mixed with an oxidative acid such as sulfuric acid to form a slurry which is then heated at a temperature higher than 100.degree. C.
With the above method, dioxins are efficiently converted into harmless substances. However, the method has problems that energy consumption is large and high apparatus costs are required, because the treatment is performed at a temperature of at least 100.degree. C. which is above the boiling point of water (at atmospheric pressure, and so forth), preferably at least 200.degree. C., while evaporating water.
It is the prime object of the present invention to provide a process for the wet decomposition of dioxins into harmless substances, which process is performed at a temperature lower than the boiling point of water and which can convert dioxins into harmless substances at a low cost.
Another object of the present invention is to provide a process for the wet processing of a flue gas from a combustion furnace containing dioxin-containing fly ash, which process is performed at a temperature lower than the boiling point of water and which can convert dioxins into harmless substances at a low cost.
The present inventors have made an intensive study for accomplishing the above objects and unexpectedly found that dioxins can be converted into harmless substances when contacted at a temperature lower than 100.degree. C. with an aqueous solution acidified with hydrochloric acid and containing a catalyst dissolved therein and have completed the present invention.
In accordance with the present invention, there is provided a process for the wet decomposition of dioxins into harmless substances, characterized in that the dioxins are contacted with an aqueous solution, acidified with hydrochloric acid and containing a catalyst dissolved therein, at a temperature lower than 100.degree. C. to decompose the dioxins into harmless substances with a decomposition rate of at least 60%.
The present invention also provides a process for the wet processing of a flue gas from a combustion furnace containing a dioxin-containing fly ash into a harmless substance, characterized in that said flue gas is contacted with an aqueous solution, acidified with hydrochloric acid and containing a catalyst dissolved therein, at a temperature lower than 100.degree. C. to cause the fly ash contained in said flue gas to migrate into said aqueous solution and to decompose the dioxins deposited on the fly ash into a harmless substance with a decomposition rate of at least 60%.
The present invention further provides a process for the wet processing of a flue gas generated from a combustion furnace, having a temperature higher than 100.degree. C. and containing a dioxin-containing fly ash into a harmless substance, characterized in that said process comprises (i) a cooling step of bringing the flue gas with gas-liquid contact with a cooling liquid to reduce the temperature of the flue gas to below 100.degree. C., (ii) a gas-liquid contacting step of bringing said flue gas obtained in said cooling step into gas-liquid contact with an aqueous solution acidified with hydrochloric acid, and (iii) a dioxin decomposing step of subjecting the fly ash-containing cooling liquid "A" obtained in said cooling step and the fly ash-containing aqueous solution "B" obtained in said contacting step, separately or jointly, to treatment conditions including a chlorine ion concentration of at least 10 mmol/liter, a copper ion concentration of at least 20 mg/liter and a treatment temperature of lower than 100.degree. C. to decompose the dioxins contained in the fly ash into harmless substances.
The present invention further provides a process for the wet processing of a flue gas generated from a combustion furnace and containing a dioxin-containing fly ash into a harmless substance, characterized in that said process comprises (i) a gas-liquid contacting step of bringing said flue gas cooled to a temperature lower than 100.degree. C. into gas-liquid contact with an aqueous solution acidified with hydrochloric acid, (ii) a fly ash concentrating step of increasing a fly ash content of the aqueous solution obtained in said gas-liquid contacting step and containing fly ash, and (iii) a dioxin decomposing step of maintaining said aqueous solution, obtained in said fly ash concentrating step and containing an increased amount of the fly ash, at a temperature lower than 100.degree. C. in the presence of a catalyst in a dissolved state, thereby decomposing the dioxins contained in the fly ash into harmless substances.
The present invention further provides a process for the wet processing of a flue gas generated from a combustion furnace and containing a dioxin-containing fly ash into a harmless substance, characterized in that said process comprises (i) a first gas-liquid contacting step of bringing said flue gas with a first treating liquid, (ii) a second gas-liquid contacting step of bringing the treated flue gas obtained in said first gas-liquid contacting step with a second treating liquid, and (iii) a dioxin decomposing step of contacting fly ash "A" captured by said first treating liquid in said first gas-liquid contacting step and fly ash "B" captured by said second treating liquid in said second gas-liquid contacting step, separately or jointly, into contact with an aqueous solution, acidified with hydrochloric acid and containing a catalyst dissolved therein, to decompose the dioxins contained in the fly ash into harmless substances.
The term "dioxins" used in the present specification is intended to refer to 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) and similar compounds thereof and to include polychlorodibenzo-p-dioxins (PCDDs) having 1-8 chlorine atoms in the dibenzo-p-dioxin structure and polychlorodibenzofurans (PCDFs) having 1-8 chlorine atoms in the dibenzofuran nucleus.
As described above, dioxins include various chlorinated compounds. Since the toxicity of dioxins varies with the kind thereof, it is necessary to establish a standard based on which toxicity of individual dioxins can be evaluated in order to evaluate a mixture of dioxins as a whole. For this reason, a factor (toxicity equivalent factor (TEF)) for calculating an amount of a dioxin in terms of an amount of 2,3,7,8-TCDD providing the same toxicity as that of the dioxin has been determined on the basis of short time toxicity evaluation of respective dioxins. By multiplying amounts of respective dioxins by the toxicity equivalent factor, toxicity equivalent quantities (TEQ) thereof can be obtained. The toxicity equivalent quantity is used for indicating a discharge amount and a concentration of dioxins.
The process for the treatment of dioxins into harmless substances according to the present invention is characterized in that the dioxins are contacted with an aqueous solution acidified with hydrochloric acid and containing a catalyst dissolved therein (hereinafter referred to simply as reaction treatment agent or aqueous solution). The treatment temperature is lower than the boiling point of water (100.degree. C.), preferably 80.degree. C. or less. The lower limit is about 30.degree. C.
The aqueous solution which is used as a reaction treating agent in the present invention has a Cl.sup.- ion concentration of at least 10 mmol (millimol), preferably at least 100 mmol, per liter of the aqueous solution. The upper limit is about 3,000 mmol. The pH of the aqueous solution is 7 or less, preferably 6 or less, with a lower limit being generally about 2. The aqueous solution can contain other inorganic acids such as sulfuric acid. In this case, it is advisable that the molar ratio [Cl.sup.- ]/[SO.sub.4.sup.2- ] of the Cl.sup.- to the SO.sub.4.sup.2- ion be 5 or more, preferably 20 or more. An upper limit is not specifically defined. As a method of contacting dioxins with the aqueous solution, there may be mentioned a method in which dioxins or dioxin-containing solids are stirred in the aqueous solution, a method in which the aqueous solution is sprayed for contacting with dioxins or dioxin-containing solids, and a method in which contact is performed in a packed column or a plate column.
The term "aqueous solution acidified with hydrochloric acid" used in the present specification is intended to mean an aqueous acidic solution containing chlorine ion. Examples of the acids used for maintaining the acidity may include hydrochloric acid, sulfuric acid and nitric acid. The use of hydrochloric acid is preferred.
The term "decomposition of dioxins" used herein is intended to refer to conversion of dioxins into detoxified compounds.
The aqueous solution acidified with hydrochloric acid used as the reaction treatment agent in the present invention contains a catalyst which serves to promote the decomposition of dioxins. The inventors' studies have revealed that the contact of dioxins with the aqueous solution can convert dioxins into harmless substances. When, however, the aqueous solution does not contain the catalyst, it takes a long time to convert dioxins into harmless substances. Thus, from the industrial and commercial point of view, it is important that the catalyst should be present in the aqueous solution. As the catalyst, a metal ion is used. The metal of such a metal ion may be, for example, iron, manganese, copper, nickel, cobalt, molybdenum, chromium, vanadium, tungsten, silver and tin. The above metal ions may be used by themselves or as a mixture of two or more thereof. The metal ion may be an ordinary metal ion or a complex ion. The use of a copper ion or an iron ion is found to be preferred as a result of the inventors' studies. The amount of the metal ion contained in the aqueous solution is not specifically limited. In the case of copper ion, the amount is 20-10,000 mg/liter, preferably 100-5,000 mg/liter, in terms of elemental copper. No increase of the effect of the addition is expected even when the amount exceeds 10,000 mg/liter. In the case of other metal ions, the amount thereof is similar to that of the copper ion.
The catalyst used in the present invention is generally in the form of a metal oxide or a metal salt, such as a chloride, an oxide, a carbonate or a sulfate. The aqueous solution acidified with hydrochloric acid and used as the reaction treatment agent for rendering dioxins harmless contains such a metal oxide or metal salt in a dissolved state. In this case, however, the catalyst can contain an undissolved matter which is generally in the course of being converted into a dissolved state. A catalyst containing such an undissolved matter in the course of being converted into a dissolved state can be effectively used.
As the catalyst used in the present invention, metal components contained in a combustion ashes such as a fly ash or a bottom ash may be utilized. Combustion ashes often contain metal components which can function as the above-described catalyst. By stirring such a combustion ash in an aqueous solution acidified with hydrochloric acid, the metal components contained therein can be utilized as the catalyst. Namely, the metal contained in the combustion ash is dissolved in the aqueous solution to form a metal ion when the combustion ash is contacted with the aqueous solution acidified with hydrochloric acid. The thus obtained aqueous solution acidified with hydrochloric acid and containing such a metal ion dissolved therein can be used as the reaction treatment agent for the wet decomposition of dioxins in the present invention.
The term "catalyst" used in the present specification is intended to include those which have a function to rendering dioxins harmless while being changed their valence. Thus, the above-described metal ion used as the catalyst does not necessarily have the same valence. The metal ion may be a mixture of a low valence metal ion and a high valence metal ion, for example, a mixture of a monovalent copper ion and a divalent copper ion. A metal ion whose valence increases or decreases during the reaction may also be used. Illustrative of the suitable reaction treatment agent is an aqueous solution acidified with hydrochloric acid and containing cuprous chloride and cupric chloride.
The aqueous solution used as the reaction treatment agent in the present invention may contain a substance (contact accelerating agent) capable of accelerating the contact of dioxins with the aqueous solution. The contact accelerating agent may include surfactants and alcohols. The kinds of the surfactants are not specifically limited. Anionic, cationic, nonionic and amphoteric surfactants may be used. The amount of the surfactant added to the aqueous solution is 0.005-1% by weight, preferably 0.01-0.5% by weight. As the alcohol, a lower alcohol such as methanol, ethanol or propanol may be suitably used. The amount of the alcohol is 0.5-10% by weight, preferably 1-10% by weight.
In the present invention, it is preferred that the reaction treatment agent be irradiated with an ultrasonic wave for the purpose of accelerating the contact between the reaction treatment agent and dioxins. Ultrasonic waves customarily used for the formation of emulsions may be suitably used for the purpose of the present invention.
If necessary, the reaction treatment agent can be contacted with oxygen or an oxygen-containing gas to enhance the concentration of oxygen dissolved therein. As the method for contacting the aqueous solution with oxygen or an oxygen-containing gas, there may be mentioned a method in which the oxygen or oxygen-containing gas is blown into the reaction treatment agent, a method in which the oxygen or oxygen-containing gas is contacted with fine droplets of the reaction treatment agent and a method in which the reaction treatment agent is brought into counter-current contact with the oxygen or oxygen-containing gas in a packed column. As the oxygen-containing gas, there may be mentioned air and oxygen-enriched air.
In the process of converting dioxins into harmless substances according to the present invention, dioxins alone are rarely subjected to the treatment. Rather, dioxins are generally treated in the state where they are deposited on solids. Examples of the solids on which dioxins are deposited include combustion ashes discharged from various kinds of combustion furnaces. The combustion ashes include fly ashes contained in flue gases discharged from combustion furnaces and bottom ashes accumulated on bottoms of furnaces. Such combustion ashes generally contain unburnt carbon (or carbonaceous materials) within which dioxins are present. Thus, it is very difficult to convert such dioxins into harmless substances. Thus, when such combustion ashes are treated with the reaction treatment agent into a harmless substance, it is preferred that the contact accelerating agent for accelerating the contact between the reaction treatment agent and the dioxins be used. Alternatively, for the purpose of accelerating the contact between the reaction treatment agent and the dioxins, it is effective to irradiate the reaction treatment agent with an ultrasonic wave or pretreat the combustion ashes by, for example, pulverization or incineration to reduce the unburnt carbon content.
When a combustion ash is used as a raw material to be treated, the combustion ash generally contains unburnt carbon or carbonaceous materials. Since the carbonaceous materials prevents the dioxins contained therein from contacting with the aqueous solution, it is difficult to convert the dioxins into harmless substances. This difficulty increases as the amount of the carbonaceous materials increases. For this reason, it is preferred that the amount of carbonaceous materials contained in a combustion ash be previously reduced. According to the studies of the present inventors, it has been found to be desired that the amount of the carbonaceous materials be 2% by weight or less, preferably 1% by weight or less, more preferably 0.5% by weight or less. To achieve this purpose, it is preferred that the combustion conditions in a combustion furnace for combusting wastes such as refuses be controlled so that the amount of carbonaceous materials contained in a combustion ash discharged therefrom is reduced. Further, in the case of a treatment of a fly ash or a bottom ash to which a large amount of carbonaceous materials deposit, it is preferred that the ash be combusted to reduce the amount of the carbonaceous materials before treating the ash into a harmless substance.
Solids having deposits of dioxins may be dioxin-polluted soils as well as the above-described combustion ashes such as fly ashes and bottom ashes.
According to the present invention, not only dioxins by themselves but also such dioxins deposited on solids may be treated into harmless substances. In this case, since the treatment temperature is lower than the boiling point of water, energy consumption is very small and the apparatus costs are low. The treatment time is about 1-100 hours. Concretely, however, the treatment time varies depending upon the treatment conditions including the state in which dioxins to be treated exist and the composition of the reaction treatment agent as well as the decomposition rate of dioxins. Thus, it is difficult to unconditionally determine the treatment time. In the present invention, it is important that the dioxin decomposition rate should be at least 60%, preferably at least 80%, more preferably at least 90%. In other words, by suitably selecting the treatment conditions, inclusive of the composition of the reaction treatment agent and treatment time, the kind of the catalyst contained in the reaction treatment agent and the pretreatment conditions for enhancing the reactivity of dioxins, etc., it is possible to attain a dioxin decomposition rate of at least 60%, preferably at least 80%, more preferably at least 90%. The process of decomposing dioxins with a decomposition rate of at least 60% by using a cheap reaction treatment agent such as an aqueous hydrochloric solution and a low treatment temperature which is greatly lower than the boiling point of water has been first developed by the present inventors.